a most simple temperature interface is germanium transistor B,K, to multimeter.... but is better to use specialized temperature sensors.
Also you have to choos scheme for drive control of the temperature swich.

other way is computer if your needs is covered.
Will be cool if you try to transfer control to computer (RS232 or USB temp sensors).

try using a digital multimeter on dc volts, supply 12 volt supply with series resistor to vary output current through temperature controlled resistor , adjust resistor to set output to multimeter and calibrate curcuit.. maybe it would work?

i`m currently making the same sort of project for my cars thermal fans , Two stage temp cut in cut out may will try using lm 4001 nand gate as it has 2 or 4 inputs and 2 outputs ,digital inputs so before the input is the analoge measurment using a resistor as a trim and a temp sensor 1000 ohm -30 to 125 degC linear , when the sensor output to the input makes the grounding resistor input to the 4001? go HIGH the output state changes makinmg the 16 pin IC give a output state of change either on or off i think up to 80ma t , then its = up to transistors and relays. Thats my sort project if that gives you and idea`s . i used the ic for simple switching and debouncing for a fish bite indicator that flashed leds when switch was triggered once 1 led would stay on and every bite after that another would flash then fade out took ages to get into the ic but i found a book in the shelve and away i went ended up using the project for local fisherman for springlines mainly at night [email protected][/url]